One type of prior art accelerometer comprises a paddle that is suspended by flexures from a support ring that surrounds the paddle. The paddle can pivot with respect to the support ring about a hinge axis that passes through the flexures. The combination of the paddle, flexures and support ring is referred to as a proof mass assembly, and may be fabricated as a unitary structure from fused quartz. The proof mass assembly is positioned between upper and lower mounting members that clamp the support ring between them, but leave the paddle free to pivot about the hinge axis.
The accelerometer includes a sensing circuit for sensing the rotational position of the paddle, and a force rebalance circuit for exerting a force on the paddle. In one type of sensing circuit, a first capacitor plate is formed on one surface of the paddle, and a second capacitor plate is formed on the adjacent mounting member. The sensing circuit measures the capacitance between these plates, to thereby determine the paddle position with respect to the mounting member. One well-known type of force rebalance circuit comprises a coil mounted to the paddle, and a magnet formed in the adjacent mounting member. When a current is passed through the coil, the current interacts with the magnetic field of the magnet to produce a force on the paddle.
In operation, the accelerometer is affixed to an object whose acceleration is to be measured. Acceleration of the object along a sensing axis normal to the paddle results in rotation of the paddle about the hinge axis with respect to the support ring and mounting members. The resulting capacitance change caused by this movement is sensed by a feedback circuit. In response, the feedback circuit produces a current that is passed through the coil, producing a force that tends to return the paddle to its neutral (i.e., unrotated) position. The magnitude of the current required to maintain the paddle in its neutral position provides a measure of the acceleration along the sensing axis.
In accelerometers of the type described above, it is generally necessary to mount a capacitor plate and a coil on at least one of the paddle surfaces. It is therefore usually desirable to maximize the area of the paddle surface for a given sized instrument. To accomplish this, the support ring surrounding the paddle is often made thin in a radial direction, to thereby maximize paddle area. Use of a relatively thin support ring also facilitates the use of certain stress isolation techniques. For example, the support ring may be clamped between the mounting members only along a section of the support ring remote from the flexures, thereby minimizing the amount of clamping stress transmitted to the flexures.
The means provided for connecting the mounting members to one another, such that they clamp the support ring, is an important feature of accelerometer design. In one known approach, the mounting members and proof mass assembly form a cylindrical sandwich structure, and are held together by a tight fitting cylindrical band that extends around the sandwich structure, and that contacts the mounting members without contacting the proof mass assembly. This "belly band" approach has the advantage that it does not require fastening elements to pass through the proof mass assembly, and thereby avoids problems with interference between the fastening elements and the paddle, sensing means and force rebalancing means.
A disadvantage of the bellyband design is that it is difficult to concentrate the clamping force on a particular section of the support ring. As described above, it may be desirable to exert all or a preponderance of the force along one side of the support ring, to avoid coupling stress into the flexures. However, bolts or other fastening members have generally not been passed between the mounting members through the proof mass assembly, because the radial thinness of the support ring would require such fastening members to pass through the paddle itself, thereby interfering with the sensing and/or force rebalance system. In addition, bolts passing through the paddle would present contamination problems, resulting from small particles trapped in the threads formed in the mounting members above and below the paddle.
In light of the above, there is a need for a fastening system for an accelerometer of the type described above, that permits tailoring of the clamping force, but that does not result in interference or contamination problems.